For patients with early stage breast carcinoma undergoing breast conservation treatment, the standard method of delivering radiation is to treat the whole breast, generally followed a boost to the primary tumor site. This standard form of radiation treatment typically lasts for 5 to 8 weeks, and achieves excellent long-term outcomes with a high rate of local control, a low rate of complications, and a high rate of acceptable cosmetic results.

Accelerated partial breast irradiation (APBI) is a newer method of radiation delivery. In contrast to standard whole breast radiation, APBI delivers an accelerated course of radiation treatment (typically in one week or less) to a small volume of breast tissue in and around the primary tumor site. APBI was developed to reduce the overall length of treatment time and to improve patient convenience. Not all potential breast conservation patients are eligible for APBI, and candidates for APBI are generally selected for favorable characteristics.

A number of different methods for delivering APBI have been described: (a) interstitial brachytherapy implant (high dose rate or low dose rate); (b) intracavitary brachytherapy (balloon catheter); (c) intraoperative radiation (orthovoltage or electrons); or (d) external beam radiation (3-D conformal, IMRT [intensity modulated radiation therapy], or protons). Each of these different methods has potential theoretical advantages and disadvantages, and none has been shown to be superior or inferior to another method. A recent study out of Massachusetts General Hospital of a small number of patients treated with APBI with protons found that there were significant acute side effects (change in skin color, skin breakdown, and rib pain), but excellent long term cosmetic outcome (100% of both doctors and patients thought the cosmetic outcome was good or excellent one year after treatment).

Another form of APBI, the MammoSite® device, was approved for use in the U.S. by the F.D.A. (Food and Drug Administration) in 2002, and therefore, only short-term outcomes (less than 5 years) after treatment have been reported for this form of APBI. The largest trial comparing MammoSite® to whole breast radiation found similar cancer control rates in the breast and survival between the two groups. However, it is important to note that this was done in a group of women with very favorable cancer characteristics and the follow up for this study is still quite short.

The best scientific method to compare a new form of treatment (APBI) to a standard form of treatment (whole breast radiation) is to perform a randomized clinical trial where patients are randomly assigned to one of two different treatment arms, and to then compare the outcomes for the two groups. Such randomized trials require thousands of patients with long-term follow-up to show whether there is a small, but meaningful difference in outcome between two forms of treatment. At the present time, at least five randomized trials have been developed or proposed. These trials use a variety of technical methods of delivering APBI, but they all use whole breast radiation as the control arm. One American trial is open and is currently accruing patients; this trial is being run jointly by the NSABP (National Surgical Adjuvant Breast and Bowel Project) and the RTOG (Radiation Therapy Oncology Group) (The planned accrual is 4300 patients, and the trial opened in 2005). Three European trials are open, and one Canadian trial is under development. The planned accrual for these five clinical trials is over 8,000 total patients. Hopefully, these randomized trials will give a more definitive answer as to the effectiveness of APBI in comparison to standard whole breast radiation treatment.